FIELD OF THE INVENTION
The invention relates to a remote-control access control device and a method for operating the device, particularly for a motor vehicle. The device has a stationary transmitter and receiver unit and a portable transponder. An unlocking unit, such as a central locking system, can be controlled with the aid of bidirectional dialog between the transmitter unit, the transponder, and the receiver unit.
Such an access control device is known from Published European Application No. 0 440 974 A1. In that device, a transponder and a transmitter and receiver unit with a question and answer dialog is used to identify a vehicle user and to unlock the vehicle if the user is authorized. Demarcation of the operative range for certain functions of the access control device is assured by providing a short range of the transmitter unit signals, while the transponder, with its wide range, can perform additional functions as well.
In any case, such a device must have recourse to a mechanical key with which the vehicle can be unlocked, if problems arise in the question and answer dialog between the transponder and the vehicle. Such problems may, for instance, be a drop below an allowable threshold in the supply voltage of the transponder or of the transmitter and receiver unit, or interference from an interfering transmitter broadcasting in the immediate vicinity.